It is a typical characteristic of automatic weapons, such as machine guns, that accuracy is relatively poor in comparison with rifles. This, chiefly, is due to better control of the rifle, involving precise aiming, followed by a single shot, with the bullet leaving the muzzle before peak forces are reached. Also, rifle feed mechanisms are relatively simple and their operation does not impose disruptive forces upon the weapon during each shot. There often are no recoiling parts in a rifle whereby the bolt may remain locked in full breech position until manuably withdrawn after each shot is fired. Since rifle shots are individually aimed and separately fired, control is firm, continuous and accurate.
The operating conditions in full automatic weapons differ considerably from rifles, and combine generally and cumulatively to result in lack of control over the precise direction of firing in regard to each round. The necessity of continuous ammunition feeding during sustained gunfire bursts usually involves use of recoil force to operate the feed mechanism. The excess recoil force beyond that required to actuate the feed system is reacted by a buffer usually having high restitution characteristics.
Restitution refers to the proportion of incoming velocity from an impacting mass which is returned to that mass. Thus, if a steel bolt carrier is impacted against a solid steel buffer plate, most of this movement will be returned to the bolt carrier in the opposite direction. A buffer system which impacts solid steel surfaces against similar surfaces has very high restitution, and consequently high recoil force impulse peaks applied through the gun buttstock to the user. Fully automatic weapons of modern design known in the prior art have a fire rate on the order of 650 to 1000 rounds per minute and apply recoil peak forces of about 500 pounds or more to the gunner through their buttstocks. This force naturally disrupts the holding force applied to the weapon by the user, severely compromising his ability to aim or otherwise control the weapon. This is especially true when recoil forces comprise a series of discrete impulses as in the case of machine guns firing a succession of rounds in each burst. The effect of this distuptive force is amplified when the force movements about the gun are asymmetrical or otherwise unstabilly oriented. Thus, if the principal vectors of the recoil force are angularly or laterally displaced away from the holding force vectors applied by the user of the weapon, such as would tend to rotate the weapon about any of its three principal axes, the resulting kinetic effects in combination will produce an inherently unstable machine gun. The same deficiencies will usually result from asymmetrical forces applied to the gun from any other cause, internal or external. Such cases illustratively include use of single cylinder systems above or below the barrel in a gas operated weapon, ammunition belt weight or pulling force widely displaced from the gun center of gravity, excessive weight of the weapon, or poor adaptation of gun supporting points with respect to body areas of the user primarily involved in supporting the weapon and resisting its recoil.
The lack of accuracy in automatic weapons is generally compensated by their rapid firing feature whereby a screen of bullets is sprayed over an area generally encompassing the target. This type of weapon is particularly effective when used in an infantry rifle squad by forcing the enemy to take cover while the squad is advancing on the enemy position. Although some of the bullets will hit the target, most will miss, depending upon the range and size of the target. Since this results in far less efficient use of ammunition, machine guns and other automatic weapons represent an increased cost over rifles for the sake of multiple hit probabilities, when a wider range of lethality is desired or multiple targets are simultaneously involved.